This invention relates to apparatus for measuring smoke density using a beam of light through which the smoke passes to attenuate the intensity of the transmitted light depending on the smoke density.
Known smoke density measuring apparatus of the aforesaid kind comprises a duct through which the smoke passes, a light source that directs a collimated beam of light across the duct to a photodetector on the opposite side, both the light source and the photodetector being housed in chambers that open into the side walls of the duct and which are supplied with air under pressure so that there is a constant flow of air into the duct to prevent the entry of smoke that might otherwise deposit smoke particles within the chambers and produce measuring errors. As a result of the air flow into the duct, the flow of smoke is modified and produces a reduction in the width of the column of smoke across the light path. The effective optical path length is therefore less than the actual duct width and is found to vary with the rate of flow of the smoke column through the duct. Therefore, in order to avoid significant measuring errors the smoke flow rate is maintained constant or at least above a minimum level below which a reduction in the flow rate has a progressively more significant effect. The problem with this restriction is that the same apparatus cannot then be used for a wide variation of smoke flow rates such as are met when measuring smoke in the exhaust of different size engines operating at different speeds.
Another characteristic of the known smoke density measuring apparatus is that at lower smoke temperatures agglomeration of the smoke particles can occur and produce lower smoke density measurements. In order to avoid such effects the known apparatus is restricted to measurements above a pre-determined minimum smoke temperature, and once again the scope of use of the apparatus is limited, at least as far as the measurement of engine exhaust smoke is concerned.
An object of the present invention is to provide smoke density measuring apparatus in which the above limitations are reduced or overcome.